1. Field of the Invention
The present invention relates generally to control systems for automatic transmissions employed in vehicles, and more particularly, to a system for controlling operations of engaging friction elements in an automatic transmission provided in a vehicle by means of oil pressure varied automatically in accordance with the operating condition of the automatic transmission.
2. Description of the Prior Art
There has been commonly used in vehicles an automatic transmission which is coupled with an engine provided in the vehicle and includes a torque converter comprising a pump impeller, a turbine runner and a stator, and a power transmitting gear arrangement including a speed change mechanism having engaging friction elements, such as clutch and brake elements, operating with oil pressure. Such an automatic transmission employed to be coupled with the engine in the vehicle is usually accompanied with a hydraulic pressure control device including an oil hydraulic control circuit for causing the engaging friction elements in the speed change mechanism to work.
When each speed change is carried out in the automatic transmission accompanied with the hydraulic pressure control device, oil pressure for causing the engaging friction elements in the speed change mechanism to work is supplied to each of the engaging friction elements through the oil hydraulic control circuit, as disclosed in, for example, U.S. Pat. No. 4,283,970 or U.S. Pat. No. 4,506,563. In the case where the speed change is completed by causing each of two engaging friction elements to be in engagement or disengagement selectively, it is required that controls for varying the oil pressure applied the respective engaging friction elements are performed individually. Further, in connection with such controls as performed individually, it is desired that the control for causing one of the engaging friction elements to be in engagement or disengagement by means of the oil pressure is carried out at an appropriate time after the control for causing the other of the engaging friction elements to be in its engaging or disengaging state by means of the oil pressure is completed so that shocks of great degree on a body of the vehicle and sudden rise in speed of an engine with which the automatic transmission is coupled, which are contingent to each speed change, are avoided.
However, in an actual automatic transmission which is employed to be coupled with an engine in a vehicle and in which controls for varying oil pressure applied two engaging friction elements are conducted individually for carrying out a selected speed change, it is feared that a time at which the control for causing one of the engaging friction elements to be in its engaging or disengaging state by means of the oil pressure is carried out after the control for causing the other of the engaging friction elements to be in its engaging or disengaging state by means of the oil pressure is completed is undesirably changed to be unsuitable owing to dispersion in characteristic of or aged deterioration in each of the engine, a torque converter contained in the automatic transmission, the engaging friction element and so on, after the time has been set appropriately once. Further, since an operating oil used for producing the oil pressure in a hydraulic pressure control device accompanying with the automatic transmission changes in viscosity in response to its temperature in such a manner that the viscosity increases when the temperature is relatively high and decreases when the temperature is relatively low, it has been quite difficult in conventional hydraulic pressure control devices to make such an arrangement that the control for causing one of the engaging friction elements to be in its engaging or disengaging state by means of the oil pressure is carried out at an appropriate time after the control for causing the other of the engaging friction elements to be in its engaging or disengaging state by means of the oil pressure is completed under both a condition in which the temperature of the operating oil is relatively low and a condition in which the temperature of the operating oil is relatively high.